The invention relates to a process for removing dust from a moving paper web, particularly a tissue web, where the air moving along with the web which has a high dust content is removed from the boundary layer. In addition, the invention relates to a device for implementing the process with a first separating box mounted across the sheet running direction.
In tissue-making, 1 to 2% of production, depending on the raw material, product, final dry content and chemical input, collects as dust in the creping sector. On the one hand, this dust has a negative effect on production and on the other, it creates a health and safety risk for the operating staff. Due to the trend towards softer tissue grades and the use of more mechanical pulp, the dust problem is increasing further. The dust comprises fine particles and fibre fragments which are removed from the paper web, primarily at the creping doctor. Some of the dust drops onto the floor of the machine room and the remainder is carried along with the air boundary layer on both sides of the paper web while it is transported from the creping doctor to the reel spool. Part of the dust remains on the surface of the paper web, which can cause difficulties further on in the finishing process.
The invention is aimed at creating a process and a device where the dust generated on a high-speed paper web, particularly a tissue web, is removed in such a way that the permissible limiting values for dust are not exceeded and, at the same time, the availability of the paper machine, particularly tissue machine, is increased.
The invention is thus characterized by the paper web running at a tangent onto and off the curved guide surface of a separating box, designed as a stabilizer, and by the air being deflected out of the boundary layer and removed by the separating box. Since the separating box is designed as a stabilizer, the web is guided over it exactly and the susceptibility to sheet breaks is substantially reduced because the web runs onto and off the curved guide surface at a tangent. This effect is further enhanced by the air being deflected out of the boundary layer, directed into the separating box and carried away from there. Thus, the vortices and overpressure otherwise common, and which also lead to sheet breaks, are avoided.
A favorable further development of the invention is characterized by the air being extracted from the boundary layer. This further reduces the risk of a sheet break due to overpressure.
A favorable configuration of the invention is characterized by the air being removed evenly over the sheet width. This measure also prevents overpressure occurring locally, which could also lead to sheet breaks, at any event with very thin paper grades.
An advantageous further development of the invention is characterized by the air being removed from the underside of the paper web in addition and then carried off, while the air removed from the underside of the paper web can be extracted by suction. By removing and then extracting the air from the underside, the dust adhering to the paper here is also removed and carried off. As a result it is possible to adhere to the required dust limiting values more easily.
An advantageous configuration of the invention is characterized by adding ambient air when the dust-filled air has been removed in order to avoid vortices from forming. In order to prevent any vortices forming while extracting sufficient air to remove the dust, and thus avoid any risk of a sheet break, dust-free ambient air is fed in at these points and the appropriate pressure thus re-established.
A favorable further development of the invention is characterized by the paper web being stabilized before the air is removed, where air carried along can be carried off by the stabilizer. This additional stabilizing of the paper web before the air is removed facilitates sheet guiding and also diminishes the risk of sheet breaks as a result. If air carried along is deflected by the stabilizer, some of the dust can be removed right away before the air separation process itself.
The invention also refers to a device for removing dust from a moving paper web, particularly a tissue web, with a first separating box mounted across the sheet running direction. It is characterized by the first separating box being designed as a stabilizer with a curved guide surface for the paper web and has a device for deflecting the air boundary layer into a collecting duct in the separating box. Since the separating box is shaped as a stabilizer with a curved guide surface, good sheet guiding is achieved and as a result, the risk of a sheet break is reduced. By carrying the air boundary layer into a collecting duct at the same time, the dust can be removed effectively from the high-speed paper web.
A further development of the invention is characterized by the cross-section of the collecting duct widening towards the drive side of the machine. This has the effect of carrying the air off evenly over the sheet width, thus preventing any vacuum or overpressure locally, which could lead to sheet breaks.
An advantageous configuration of the invention is characterized by a suction slot, which should preferably be adjustable, being provided on the side of the separating box on which the paper web runs onto its surface. The dust-filled air can be removed from this area through the suction slot, with the adjusting facility being used to either remove or extract whatever amount of dust is generated.
A favorable further development of the invention is characterized by the separating box being able to be opened along its entire width. This design provides an easy means of cleaning the box, which is particularly important with dust-filled air in a humid environment.
An advantageous configuration of the invention is characterized by a further separating box being provided on the underside of the paper web and onto which the paper web runs at a very narrow angle, preferably between 1 and 5 degrees, for example from 1 to 2 degrees. By placing a further separating box on the underside of the web it is possible to remove additional dust. Running the web onto the box at a very narrow angle is an easy method of achieving better sheet guiding, thus reducing sheet breaks on sharp edges.
An advantageous further development of the invention is characterized by the additional separating box having a deflection plate, preferably of swivelling design. This deflection plate can be used to guide the air directly into the separating box, while the swivelling design makes it possible to set the amount of air to be removed.
A favorable configuration of the invention is characterized by the bottom separating box being divided into at least two chambers. With this design the air upstream and downstream of the separating box can be carried off separately and the amount to be removed can also be set separately in order to guarantee stable sheet guiding without breaks.
A favorable further development of the invention is characterized by a stabilizer being provided upstream of the first separating box, which has the effect of spreading the paper web. The stabilizer can be of swivelling design. This additional stabilizer provides even more stable sheet guiding, while also generating additional air deflection and thus, reducing dust levels. With a swivelling design the ideal web tension is always guaranteed and if there is a sheet break, this stabilizer can be swung out of the way before the web is fed in again.
An advantageous further development of the invention is characterized by a funnel-shaped suction hood being provided at the doctor area on the drive side of the paper machine. With an extraction facility of this type it is also possible to remove the dust occurring during a sheet break and new web feed, thus reducing the dust loading.
An advantageous configuration of the invention is characterized by a further separating box being provided on the top side of the paper web. By including this box it is also possible to remove any residual dust adhering to the top side of the paper web before it is wound onto the reel spool.